This invention was made in the course of, or under a contract with the United States Energy Research and Development Administration, or its predecessor the United States Atomic Energy Commission.
Thallium-201 is a potentially useful radioisotope for various medical applications, including myocardial visualization and possible assessment of physiology, as a renal medullary scanning agent, and for tumor diagnosis.
Since Tl.sup.+ is a good biological analog of potassium, it has the biological advantages of K.sup.+ including high extraction by the myocardium on a single circulation. Furthermore, it has been observed that thallium activity remains in the myocardium even up to 18 hours postinjection. This removes a disadvantage of potassium from thallium, namely the rapid leakage of potassium from the myocardium. The ability to take many views may be crucial if it is necessary to view small infarcts in profile in order to visualize them. Delayed scans may yield improved resolution and the ability to look for leakage of thallium from the myocardium over several days of observation for myocardial infarction.
.sup.201 Tl, with a 73 hour half-life, decays by electron capture mainly to the ground state of stable mercury-201. It emits mercury K-X-rays of 69-83 keV in 93% abundance, and photons of 135 and 166 keV in 10% total abundance. The photons of .sup.201 Tl are detected with high efficiency and resolution in a low-energy collimator, gamma camera detection system.
Furthermore, the 73 hour half-life gives .sup.201 Tl a good shelf life, which is not only more convenient (by a considerable amount, in the cases of isotopes with half-lives of a few hours) than many of the other radioisotopes considered for myocardial visualization, but is also invaluable for availability for emergency use. The half-life of .sup.201 Tl is consistent with a weekly shipment of this radiopharmaceutical from its supplier.
In view of the foregoing there has been interest in developing a practical and economical method of preparing high purity and specific activity thallium-201. Previous methods did not produce the thallium-201 in sufficient purity. A typical previous method was based on a formation of insoluble lead chloride, which as mentioned did not give the desired purity.